The invention relates to novel metallocenes containing aryl-substituted indenyl derivatives as ligands which can be used very advantageously as catalysts components in the preparation of polyolefins of high isotacticity, narrow molecular-weight distribution and very high molecular weight.
Polyolefins of high molecular weight are of particular importance for the production of films, sheets or large hollow articles or moldings, such as, for example, pipes.
The literature discloses the preparation of polyolefins using soluble metallocene compounds in combination with aluminoxanes or other cocatalysts which, due to their Lewis acidity, are able to convert the neutral metallocene into a cation and stabilize it.
Soluble metallocene compounds based on bis(cyclopentadienyl) dialkylzirconium or bis(cyclopentadienyl) zirconium dihalide in combination with oligomeric aluminoxanes are capable of polymerizing ethylene in good activity and propylene in moderate activity. Polyethylene having a narrow molecular-weight distribution and moderate molecular weight is obtained. The polypropylene prepared in this way is atactic and has a very low molecular weight.
The preparation of isotactic polypropylene is achieved with the aid of ethylenebis(4,5,6,7-tetrahydro-1-indenyl)zirconium dichloride together with an alumin-oxane in a suspension polymerization (cf. EP 185 918). The polymer has a narrow molecular-weight distribution. A particular disadvantage of this process is that, at industrially relevant polymerization temperatures, only polymers having a very low molecular weight can be prepared.
A special preactivation method for the metallocene using an aluminoxane has also been proposed, resulting in a significant increase in the activity of the catalyst system and in a considerable improvement in the grain morphology of the polymer (cf. DE 37 26 067). However, the preactivation hardly increases the molecular weight at all.
Also known are catalysts based on ethylenebisindenyl-hafnium dichloride and ethylenebis(4,5,6,7-tetrahydro-1-indenyl)hafnium dichloride and methylaluminoxane, by means of which relatively high-molecular-weight polypropylenes can be prepared by suspension polymerization (cf. J. Am. Chem. Soc. (1987), 109, 6544). However, the grain morphology of the polymers produced in this way under industrially relevant polymerization conditions is unsatisfactory, and the activity of the catalyst systems employed is comparatively low. Together with the high catalysts costs, inexpensive polymerization using these systems is thus impossible.
A significant increase in the molecular weight has been achieved by using metallocenes in which the aromatic .pi.-ligands fixed by a bridge carry substituents in the 2-position (cf. DE 40 35 886) or in the 2- and 4-position (cf. DE 41 28 238).
A further increase in the molecular weight has been achieved by using aromatic .pi.-ligands containing substituents in the 2-, 4- and 6-position (cf. DE 41 39 596) and aromatic .pi.-ligands of the 4,5-benzoindenyl type (cf. DE 41 39 595).
The last-mentioned metallocenes containing said substituents are already very effective in this respect at the polymerization temperature of 70.degree. C. Nevertheless, the molecular weights which can be achieved at the industrially optimum polymerization temperature of 70.degree. C. are still too low for many industrial applications, such as, for example, the preparation of polymers for pipes and large hollow articles, and in particular fibers.
Under the constraints of inexpensive large-scale production, polymerizations must be carried out at the highest possible reaction temperature, since the heat of reaction produced at relatively high polymerization temperatures can be dissipated using little cooling medium. The cooling-water circuit can therefore be made significantly smaller.
A disadvantage which frequently occurs in soluble (homogeneous) metallocene/methylaluminoxane catalyst systems in processes in which the polymer is formed as a solid is the formation of thick deposits on reactor walls and stirrer. These deposits are formed by agglomeration of the polymer particles if the metallocene, or aluminoxane, or both, are in the form of a solution in the suspension medium. Deposits of this type in the reactor systems must be removed regularly, since they rapidly achieve considerable thicknesses, have high strength and hinder heat exchange with the cooling medium.
It is therefore advantageous to employ metallocenes in supported form. An efficient and simple process for supporting metallocenes which can be employed universally in all polymerization processes has been proposed (cf. EP 92 107331.8).
A further disadvantage in the case of stereospecific polymerization of prochiral monomers, for example of propylene, using metallocene catalysts is the relatively low isotacticity, which results in low melting points in the case of isotactic polypropylene. In particular metallocenes containing substituents in the 2- and 4-position and specifically rac-dimethyl-silylbis(2-methyl-4-isopropylindenyl)zirconium dichloride in combination with methylaluminoxane gives, in the case of propylene, a polymer of high isotacticity and thus high melting point (cf. DE 41 28 238). Nevertheless, the melting points which can be achieved are too low at industrially relevant polymerization temperatures (for example 70.degree. C.) for some industrial applications.
However, there are also industrial applications in which low melting points are desired.
The object was to find a process and/or a catalyst system which produces polymers of very high molecular weight and, in the case of isospecific polymerization of prochiral monomers, polymers of high isotacticity in high yield. The use of a support would prevent the disadvantages known from the prior art caused by deposit formation and a high proportion of fine particles. The use of hydrogen as molecular weight regulator should then enable the entire range of industrially interesting molecular weights to be covered by means of only a single metallocene.